In recent years, image sensors such as CSP have been becoming smaller, thinner, and lighter. The sensor part of each of those image sensors was conventionally protected with a resin package. However, in recent years, a mode of protecting the sensor part by bonding a glass substrate on a Si chip is being adopted in order, for example, to make the image sensors much smaller.
In addition, the glass substrate has also been required to be much thinner in order, for example, to make a device smaller, and a glass substrate having a thinner thickness (for example, a glass substrate having a thickness of 0.5 mm or less) is being adopted.
Besides, an alkali-free glass substantially free of an alkali metal oxide is usually used for the glass substrate in order to prevent alkali ions from diffusing in a semiconductor substance formed into a film during a heat treatment process (see Patent Document 1).